Polymer particles impregnated with metal in the zero valence state are described in U.S. Pat. No. 4,197,220. The particles disclosed therein are, for example, hydrophilic and microporous polyvinylpyridine particles which are impregnated with metal by complexing with metal ions, followed by a reduction with a water-soluble reducing agent such as a borohydride. The microporosity of the support, and the state of division of the metal precipitated inside the pores, render such particles an advantageous material for the catalytic hydrogenation of olefins. Other applications are also mentioned, such as X-ray electron probe methods (since such particles are electron-dense) and protein separation techniques.
The disadvantage possessed by such particles arises from the fact that they are porous and contain metal at the surface.
In applications involving biological substances (such as proteins), cells or drugs, it is very important that the metal be completely encapsulated in the polymer of the particle and absent from the surface.
Thus, in enzymatic engineering, it is known that certain enzymes are inhibited on contact with metal ions originating from the support. For example, it has been shown that chymotrypsin immobilized on nickel powder rapidly loses its activity (Munro et al., Nonporous Magnetic Materials as Enzyme Supports: Studies with Immobilized Chymotrypsin, XIX Biotechnology and Bioengineering 101-124 (1977)).
Another undesirable effect relates to the phenomena of irreversible adsorption of hormones (T3, T4) or steroids on magnetic supports based on iron oxide, which adsorption interferes considerably with assays of the radioimmunoassay (RIA) type.
Finally, in cell culture, the presence of metal ions derived from the support may also disturb cell adhesion.
The preparation of metal colloids in an organic medium has been known since the work of Hess and Parker (Hess et al., Polymers for Stabilization of Colloidal Cobalt Particles, 10 J. Appl. Poly. Sci. 1915-1927 (1966)) and J. R. Thomas (U.S. Pat. No. 3,228,881), as well as T. W. Smith (U.S. Pat. Nos. 4,252,671 through 4,252,678, inclusive). The technique involves thermally decomposing an organo-metallic coordination compound, particularly a metal carbonyl compound, in an organic solution of polymer. A colloidal dispersion is obtained for example, a ferro-fluid comprising 10.sup.-6 to 10.sup.-5 mm metal particles in an organic solvent. The polymer is adsorbed at the surface of the metal particles, and provides for the colloidal stability of the suspension. When such a ferro-fluid is exposed to the ambient atmosphere, the metal particles, although confined in a hydrophobic organic phase, oxidize rapidly and lose their magnetic properties (Griffiths et al., The structure, magnetic characterization, and oxidation of colloidal iron dispersions, 50(11) J. Appl. Phy. 7108-7115 (Nov. 1979)). Moreover, as a result of their size, generally less than 300 angstroms, these metal particles, on which the polymer has a surfactant effect, cannot be separated from the organic solvent (by magnetization, in the case of an iron or cobalt ferro-fluid) without destroying the colloidal stability. Finally, these particles of metal, subjected to a surfactant effect, are completely incompatible with aqueous media, and accordingly cannot be used in the applications described above.
European Pat. No. 38,730 describes latices of particles of hydrophobic vinylaromatic polymer containing a magnetizable filler. These particles are obtained by dispersion of the magnetic filler in an organic phase composed of an organo-soluble initiator, the vinylaromatic monomer and/or a water-insoluble organic compound, followed by mixing of the dispersion with an aqueous solution of emulsifier, homogenization and, finally, polymerization. Such a process has the disadvantage that it can be used only for the preparation of latices of unsized particles--that is, particles possessing a spread particle size distribution.
When used as a solid phase in biological applications, the particles of such latices should preferably be sized.